The extrusion of polymer is a common processing step in polymer production. In general, polymer powder may be metered at a desired rate in to a feed hopper, then passed by gravity from said feed hopper through what is known as a feed port and into an extruder barrel in which there are rotating screws. The screws rotate to move the polymer away from the feed port along the barrel. Thermal and mechanical energy (from the screw) are provided to the polymer powder in the barrel causing it to melt. Once melted the polymer is passed into a suitable die and extruded to form polymer in the desired extruded form, such as pellets or film.
In industrial scale extrusion processes typical extruders are made up of barrel sections having a “standard” barrel length. Typically the overall length of a standard barrel section is 2 to 3 times the diameter of the screw or rotor. Conventionally, at industrial scale polymer is fed into the first barrel section in a sequence, with at least a second barrel section being provided “downstream”. At least a third barrel section is also common to ensure polymer is completely melted prior to extrusion. There are a number of parameters which are important in the extrusion process, including, for example, barrel dimensions, screw speed, screw pitch and torque. The general direction in the art has been to increase extrusion throughput, which has involved increasing all linear dimensions of the barrel, the screw speed and the torque as much as possible since these have, to date, been the limiting factors.
The principal requirement of the feeding of the polymer has been to sufficiently fill the barrel as the polymer is moved along it. The polymer is fed to the first barrel section, usually from a hopper, through an area known as the feed port. It is typical that the feed port width is maximised based on the barrel section width i.e. up to the extremity of the screws, for example up to 2 times the diameter of the screws or rotors in the case of twin screw or twin rotor extruders.
Typically, the feed port also has an axial length of up to 2 times the diameter of the screw or rotor in this section of the extruder, giving a feed port area with length and width both up to 2 times the screw diameter.
The feed rate of the polymer powder for twin-screw extruders is generally controlled by the rate at which the powder is metered into the hopper. The hopper is generally considered to run “empty” by which is meant without significant volume of powder therein and without “hold-up” of the powder at the base of the hopper. To date the feed port area has not generally been limiting on the process, and therefore has not caused a great deal of concern.